The present invention relates to an engine idling control apparatus which judges an engine idling state from an output signal of a throttle sensor.
During an idling operation of an engine, the engine speed is set to a predetermined value by regulating the area of the bypass with an idle speed control valve (ISCV), and the ignition timings are set to have a predetermined lead angle.
An engine idling operation has been mechanically judged by an idling switch. If the switch is ON, it is unconditionally judged that the throttle valve is closed, i.e., in an idling state, and the engine is set to the idling state.
In practice, however, as shown in FIG. 8, since it is necessary to certainly turn ON the idle switch when the throttle valve is fully closed, the idle switch is arranged to turn ON (on the basis of which the throttle valve is mechanically judged as fully closed) at an opening degree (.theta.0) before the throttle valve is fully closed. As a result, if the acceleration pedal is depressed to start the engine, although the engine speed rises, the idling control is not released until the throttle valve is opened to the predetermined opening degree .theta.0. Further, if the acceleration pedal is released during a deceleration operation, although the throttle valve is still open, the idling control starts when the throttle valve opening degree takes the value .theta.0.
In case of the conventional engine, under such conditions that the idle switch does not turn OFF (which is judged as a fully closed state of the throttle valve) even if the valve is still open, and that the idle switch turns ON before the valve is fully closed, an area change ratio of a throttle bore and hence a changing range of intake air quantity are small so that the starting, acceleration and the deceleration performance do not improve largely.
Latest engines, however, have a throttle with a large bore diameter so as to obtain a high performance and high output power. Even a small change in throttle valve opening degree will therefore result in a large change in intake air quantity. If the idling control is continued under the condition that the throttle valve is opened from the fully closed state and the engine speed rises, the start running and acceleration performance is degraded because of insufficient power. Further, if the idling control starts in response to an output signal from the idle switch before the throttle valve is fully closed, the deceleration performance is degraded. Not only the start, acceleration and deceleration performance are degraded, but also a proper air-fuel ratio control is not possible and an exhaust gas emission and fuel consumption are deteriorated. The controllability of idling operation is also decreased largely.
In order to deal with the above problems, it can be considered that the idle switch may be set near a fully closed position of the throttle valve so as to ensure a proper ON/OFF. However, the distance between switching contacts has the limit and setting such distance is practically difficult, so that this arrangement is not suitable for mass production and is hard to be realized.
Apart from the above, there is disclosed in, e.g., Japanese Patent Publication No. 63-15467, an idling control technique wherein the minimum value of a throttle opening degree is updated in accordance with an output signal from a throttle valve sensor, and stored in a memory. When the output value of the throttle valve sensor becomes as small as the minimum value stored in the memory, it is judged as the fully closed state (idle state) of the throttle valve.
According to this related technique, even during the OFF state of the idle switch, a fast idle mechanism operates to forcibly open the throttle valve to thereby allow the idling control. It has also been considered to apply this technique to an engine using ISCV as the fast idle mechanism to correct the physical displacement between the idle switch and throttle valve opening degree, i.e., correcting with software the condition that even the idle switch is ON, the throttle valve is actually open. This correction is so complicated that it is hard to be realized in practice.
Further, if the output value of the throttle valve sensor exceeds the stored minimum value even by one bit (minimum resolution), it is judged as the non-idling state. It is therefore likely to occur hunting between idling control and non-idling control relative to the minimum value. The controllability of not only idling operation but also air-fuel ratio is deteriorated. Furthermore, if the output value of the throttle position sensor lowers largely due to disturbances, this value is stored as the minimum value so that the idle state is hardly judged thereafter.